Aqueous fluoropolymer dispersions, when applied by such a method as coating or impregnation, can form films, coating films and the like excellent in such characteristics as chemical stability, nonstickiness and weather resistance. Therefore, they are widely used in such fields of application as lining of cooking utensils and pipes and manufacture of impregnated glass cloth membranes. Aqueous fluoropolymer dispersions are generally obtained by polymerization in the presence of a fluorinated surfactant. From the viewpoint of the characteristics of films and so forth, it is preferred that the content of the fluorinated surfactant be low.
As a method of reducing the fluorinated surfactant level, the method which comprises passing a fluoropolymer dispersion, with a nonionic, anionic or cationic surfactant added for the purpose of stabilization, through a semipermeable ultrafiltration membrane to thereby increase the concentration of the fluoropolymer has been proposed (cf. Patent Document 1).
In Patent Document 1, mention is made of sulfosuccinic acid esters or salts thereof and like anionic surfactants as stabilizing emulsifiers but there is no substantial description thereof; further, there is no substantial description about the use thereof with a nonionic surfactant.
It has been disclosed that even when an aqueous fluoropolymer dispersion contains a nonionic surfactant, the concentrate derived therefrom encounters the problems of increased viscosity and reduced stability if the content of a fluorinated surfactant with a molecular weight lower than 1000 is lower than 0.025% by weight of the solid matter and that the problems are alleviated by the dispersion containing an anionic non-fluorinated surfactant or fluorinated anionic surfactant (cf. Patent Document 2).
However, in Patent Document 2, there is no description of any sulfosuccinic acid ester as the anionic non-fluorinated surfactant usable for lightening the increase in viscosity; further, the anionic fluorinated surfactant is restricted to one with a molecular weight of not lower than 1000.
A method of concentrating aqueous fluoropolymer dispersions which comprises adding a nonionic non-fluorinated surfactant, a fluorinated anionic surfactant or a mixture of these in an amount of 10 to 5000 ppm relative to the fluoropolymer solid matter before or after the concentration procedure has also been proposed (cf. Patent Document 3). It has been pointed out, however, that this method has a problem in that low surfactant addition amounts rather cause increases in viscosity.    Patent Document 1: Japanese Kokai Publication Sho-55-120630    Patent Document 2: United States Patent Application Publication 2004/186219    Patent Document 3: United States Patent Application Publication 2004/171736 ([0016])